What type of joint is characterized by immobility due to the fusion of bones?

The type of joint characterized by immobility due to the fusion of bones is a fibrous joint. Fibrous joints are held together by dense connective tissue, including collagen, which forms a strong bond between the adjacent bones. Because of this dense fibrous tissue, these joints do not allow for any movement, making them immovable. A common example of a fibrous joint is the sutures of the skull, where the bones are tightly fused together to protect the brain.

In contrast, the other joint types mentioned allow for varying degrees of movement. Hinge joints, such as the elbow and knee, permit motion in one plane (flexion and extension). Ball-and-socket joints, found in the shoulder and hip, provide a wide range of motion in multiple directions. Pivot joints, like the one found between the first and second vertebrae in the neck, allow for rotational movement. Therefore, the distinctive characteristic of fibrous joints being immobile due to the union of bones clearly defines why this is the correct answer.